A polarizing plate having a function to transmit or shield light is a fundamental constituent element of a display device such as a liquid crystal display (LCD), along with liquid crystals which have a function of switching light. The area of application of this LCD has expanded broadly from small items such as an electronic calculator, a watch, and the like in the early day to a notebook computer, a word processor, a liquid crystal projector, a liquid crystal television, a car navigation system, indoor and outdoor measurement instruments, and the like. Also, the LCD is used in broad conditions from low to high temperature, from low to high humidity, and from low to high light intensity. Thus, a polarizing plate having high polarizing performance and excellent durability is desired.
At present, a polarizing film is manufactured by dyeing a polarizing film substrate with or incorporating therein iodine or a dichromatic dye as a polarizing element, wherein the substrate is a stretched and oriented film of polyvinyl alcohol or its derivative, or an oriented film of polyene prepared by dehydrochlorination of a polyvinyl chloride film or dehydration of a polyvinyl alcohol film. Among these, an iodine polarizing film which uses iodine as the polarizing element exhibits superior initial polarizing performance. On the other hand, this polarizing film is weak to moisture and heat, and when it is used for a long time under conditions of high temperature and high humidity, there arises a problem of durability. In order to improve durability, methods such as treatment of a polarizing film with formalin or an aqueous solution containing boric acid, use of a polymer film of low moisture permeability as a protective film, and the like are considered. However, the effects of these methods are not satisfactory. On the other hand, a dye-containing polarizing film comprising a dichromatic dye as a polarizing element has better humidity resistance and heat resistance than an iodine polarizing film, but, generally, initial polarizing performance of the dye-containing polarizing element is insufficient.
In a neutral color polarizing film produced by adsorbing several dichromatic dyes to a polymer film followed by orientation, if there is light leakage (color leakage) of a specific wavelength in the wavelength range of visible light, in a state (the perpendicular position) that two polarizing films are superimposed on each other in such a way that their orientation directions are perpendicular to each other, the hues of the liquid crystal display may change in the dark state when the polarizing films are fitted to the liquid crystal display panel. Thus, in order to prevent the color change of a liquid crystal display due to color leakage of a specific wavelength in the dark state when a polarizing film is fitted to a liquid crystal display device, it is necessary to uniformly lower the average light transmittance at the perpendicular position (perpendicular average light transmittance) in the wavelength range of visible light.
Further, in a case of a color liquid crystal projection display, namely, a color liquid crystal projector, a polarizing plate is used for a liquid crystal image-forming part. In this application, the iodine polarizing plate was used formerly, which has good polarization performance and exhibits neutral gray color. However, as mentioned above, the iodine polarizing plate has a problem that its light resistance, heat resistance, and wet heat resistance are insufficient, because iodine is a polarizer. In order to solve this problem, a neutral gray polarizing plate using a dye-containing dichromatic colorant as a polarizer has come to be used. In a neutral gray polarizing plate, colorants of three primary colors are generally used in combination in order to improve transmittance in the entire wavelength range of visible light and polarization performance averagely. Thus, there is a problem that to the demand of the marketplace for more brightness as in the color liquid crystal projector, the transmittance is still poor, and in order to realize brightness, it is necessary to increase intensity of the light source. In order to solve this problem, three polarizing plates corresponding to three primary colors, namely, plates for each of the blue channel, the green channel, and the red channel have come to be used.
Decrease in brightness cannot be avoided because light is absorbed considerably by the polarizing plate, and an image of such a small area as 0.5 to 3 inches is magnified to about several tens to one hundred and tens of inches. Therefore, as the light source, one of high luminance is used. Furthermore, desire for further increase in brightness of a liquid crystal projector is strong and, as a result, the intensity of the light source used is inevitably growing stronger. Along with this, the amounts of light and heat which the polarizing film receives are increasing.
Examples of the dyes used for production of the above-mentioned dye-containing polarizing films include water-soluble azo compounds described, for example, in the Patent Document 1 to Patent Document 6.
However, conventional polarizing plates containing the water soluble dyes have not yet satisfied the market needs sufficiently in terms of polarization characteristics, the range of absorption wavelength, hues, and the like. Furthermore, among three polarizing plates corresponding to three primary colors for a color liquid crystal projector, namely, the plates for each of the blue channel, the green channel, and the red channel, none is good in all aspects of brightness, polarization performance, durability under conditions of high temperature and high humidity, and resistance to prolonged irradiation of light. Improvement is thus desired.    Patent Document 1: JP-A-2003-215338    Patent Document 2: JP-A-2004-338876    Patent Document 3: Japanese Patent No. 2,622,748    Patent Document 4: JP-A-60-168743    Patent Document 5: JP-A-2001-33627    Patent Document 6: JP-A-2002-275381    Patent Document 7: JP-A-05-295281    Non-Patent Document 1: “Senryo Kagaku (Dye Chemistry)” written by Yutaka Hosoda